Tree holes in a mixed broad-leaf–podocarp rain forest, New Zealand

Despite the ecological importance of tree holes as habitat for many species in New Zealand, few studies have quantified the abundance, distribution or structural characteristics of tree holes in native forests. We recorded a total of 364 tree holes in ground-to-canopy surveys on 50 trees of five endemic species in the families Fagaceae and Podocarpaceae within Orikaka Ecological Area, Buller District, New Zealand. Tree holes were not uniformly distributed throughout the forest, with more holes in the three podocarp species Prumnopitys ferruginea, P. taxifolia and Dacrycarpus dacrydioides than in Nothofagus fusca or N. menziesii. However, N. fusca had the largest tree holes of any of the tree species sampled. Tree-hole volume and tree-hole opening both increased with tree size. Tree-hole opening was strongly positively correlated with internal volume, especially for the podocarps. It therefore potentially provides a useful surrogate measure for quantifying the abundance of large tree holes from cost-effective ground surveys. We estimated there might be as many as 771 tree holes per hectare of forest at this site, but fewer of these were thought to be large enough for obligate hole-dwelling vertebrate species. Our findings differ from those of previous researchers in native forests around New Zealand because earlier studies use ground-based surveys to sample the abundance of tree holes suitable for specific fauna, whereas we used climbed inspections where all tree holes were recorded. This could be particularly important for the conservation management of native vertebrate and invertebrate species.